Chirped-pulse amplification (CPA, also sometimes referred to as a regenerative amplification) has allowed the high energy storage capacity of solid-state gain media to be exploited for the generation of ultrashort laser pulses with extremely high peak intensities (by ultrashort we mean pulses whose pulsewidth at Full Width Half Maximum, FWHM, is substantially below 1 nanosecond). By stretching in time (frequency-chirping) the input low peak power seed pulse with a dispersive delay line (i.e. a stretcher), it is possible to reduce the peak intensity during amplification, thereby eliminating the deleterious and often catastrophic effects of the nonlinear interaction of the pulse with the reflective and/or refractive material of the system. After amplification, the stretched-pulse is recompressed by propagation through another dispersive delay line, which provides a frequency-chirping of opposite sign (i.e. a compressor) and consequently recompresses the pulse. The current CPA state-of-the-art can produce pulses with peak powers in excess of 50 TW and amplified pulse widths below 40 femtosecond.
Since 1985, chirped-pulse amplification (CPA) systems have matured to the point where they have become the most popular means to generating energetic short pulses. This advance is largely credited to our ability to carefully control higher order (non-linear) dispersion of the chirped pulse (these effects exist in multipass amplifiers, or system designs that incorporate elements of both, as well as single-pass power amplifier stages. The subject invention relates to all these types of amplifiers, and, unless specifically stated, the term amplifier is intended to be inclusive of all of them). For many applications it is critically important that the amplified and recompressed pulse shape be a replica of the input pulse shape. More specifically one wants to avoid pulse wings or satellite pulses that are temporally shifted with respect to the main pulse. This "pulse fidelity" is affected by several factors: amplified spontaneous emission, system bandwidth limitations, and high-order phase errors introduced by the stretching and compression process.
Amplified spontaneous emission and bandwidth limitations not associated with the stretcher and compressor design are important concerns in laser system engineering but are not the object of this invention. The focus of this invention is the effect of higher-order phase errors that arise in the stretching and compression process and our ability to compensate for them. The stretcher and the compressor alter the pulse length through the frequency-dependent phase shifts they impose on the pulse. This phase function can be Taylor expanded about the central frequency of the laser: EQU .phi.(.omega.)=.OMEGA..sub.o +.phi..sub.1 (.omega.-.omega..sub.o)+.phi..sub.2 (.phi.-.phi..sub.o) .sup.2 +.phi..sub.3 (.phi.-.phi..sub.o).sup.3 +.phi..sub.4 (.phi.-.phi..sub.o).sup.4
The zero- and first-order terms are the overall phase shift and group delay of the pulse, respectively, and do not affect the pulse width. The second-order term, .phi..sub.2, is the group-delay dispersion and produces a linear chirp on the pulse. It is the largest non-negligible term in the phase function and produces the stretching or compression of the pulse. The next two higher-order terms, .phi..sub.3, (the so-called cubic term) and .phi..sub.4, (the so-called quartic term) produce a nonlinear chirp, or distortion of the pulse, and it is the incomplete cancellation of these higher-order terms that most significantly reduces the pulse fidelity (and creates temporal wings or satellites on the output pulse). The Taylor expansion shows clearly that, the larger the bandwidth (.phi.-.phi..sub.o), the more important the contribution of the higher order terms. As the pulse width falls below 100 fs it becomes increasingly important to be able to control the higher order non-linear contributions, especially the cubic and quartic terms.
Imaging errors (aberrations) in the stretcher may cause a mismatch between the stretcher and compressor phase functions. These errors may be partially eliminated by using an imaging system that compensates both spherical and chromatic aberration. However, because the phase function of the material in the amplifier chain does not match the phase function of the compressor, the phase shift produced during amplification cannot be completely compensated by a simple change in the compressor length. Thus, a perfectly matched stretcher and compressor will not provide the best compressed pulses because the design is unable to compensate for the material of the amplifier chain.
The basic requirement that the total frequency dependent phase shift of the pulse by the laser system be zero indicates that some method must be found to compensate for the higher-order terms in the phase function--principally the cubic and quartic terms. Through ray tracing, Lemoff and Barry in "Quintic phase-limited, spatially uniform expansion and recompression of ultrashort optical pulses," Optics Letters Vol. 18 No. 19 Oct. 1, 1993 have found that they were able to design a stretcher and compressor that can compensate up to quartic phase. They achieved this by changing the input angles of the compressor (with respect to the input angle of the stretcher) and including the correct length of material in their optical chain. Although this seems to work, their design requires extremely precise alignment of the entire optical chain and is somewhat inflexible to changes in the laser system. Other designs reported in the literature have similar limitations, which are beyond the scope of the presentation here.
The designs proposed to date are plagued by serious deficiencies. An ideal stretcher-compressor system would utilize high reflectivity gratings for their broad band efficiency and stretching-compressing capability. Is should also be able to compensate (or at least minimize) cubic and quartic phase errors, be free of spatial chirp, possess relaxed alignment tolerances, and be compact. This invention meets these requirements.